Selenium rectifier



Patented Apr. 20, 1943 SELENIUM nnotmma Ferdinand Waibel, Berlin-Charlottenburg, and Erich Nitsche, Berlin-Falkcnsee, Germany, assignors to Westinghouse Electric 8; Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application April 19, 1941, Serial No. 389,470. In G Claims.

Our invention relates to selenium rectifiers,

. ance of such rectifiers.

For dry type rectifiers, such for example as the ermany June 30, 1939 selenium rectifier, it is very important that the forward-resistance be as low as possible and the back-resistance high. The rectification of low alternating current voltages is very important for use in electrical measurement as well as in the most important power applications. The resistance in the direction of fiow is dependent upon the resistance of the semi-conducting layer, governed to a certain extent by the resistance voltage drop, the trend of the current-voltage curve of the back-resistance layer and the transsition resistance of the semi-conducting surface to the supply electrode. This transition resistance must be as low as possible and must be independent of voltage, if the operation of the rectifier is to be wholly desirable. The characteristic resistance curve of the blocking layer itself is dependent upon its chemical and physical composition. Within the normal range of operation, this resistance curve should have as abrupt a drop as possible, even at low voltages. On the other hand, the resistance of the blocking layer should be as high as possible. The

magnitude of all these resistances in both directions of current flow is primarily determined by the conductivity of the selenium layer itself. In order to" facilitate high current fiow, this conductivity must be as high as possible. In addition, it is desirable to hold the thickness of the layer to a minimum of a few tenth-millimeters to one tenth-millimeters to one tenthmillimeter. However, care should be taken that in spite of the thinness of the layer it is kept free from bare or'thinned out areas, so as to avoid short circuits-between adjacent electrodes or overheating of the areas when current flows. Nevertheless, a blocking layer of high ohmic value must be produced by the semi-conducting layer. The blocking layer must withstand high voltages.

The present invention deals with the particularly effective solution of the main difficulties surrounding the production of selenium rectifiers. It deals especially with the reproducible manufacture of a selenium rectifier upon a thin metal base, such as aluminum. The rectifier on a light metal base has the technical advantage of being particularly light in weight and cheap, and it is well capable of radiating the heat infore, withstand a higher load. Thus material and weight are again saved. In addition, the operating range can be selected, with regard to the voltage stress, so that it lies within a range where the difierence between the forward and back current is particularly large.

The conductivity of selenium can be materially increased by a thorough conversion into a crystalline form by annealing it at a temperature close to the melting point. Likewise the conductivity of selenium can be increased by the addition of materials, such as pulverized halogen salts of heavy metals, alkalies, carbon powder or organic compounds. After conversion the additional materials either appear as undecomposed matters, having a certain conductivity, imbedded in the selenium or they are mainly decomposed into oxides, or selenides of good conductivity. These .again are conductive bodies and form dangerous shunting paths in the selenium and in the blocking layer itself. Such conductive foreign bodies are a source of trouble since they make the selenium appear to have good conductivity, and they can become extremely harmful during operation of the rectifier by their tendency to produce dielectric breakdown,

- particularly as a result of electrolytic processes.

The above is particularly important in connection with the reliability of the rectifier. When it is intended the oxygen shall be produced from the materials added during the annealing of the rectifier, the first consideration is to cause these oxides to act mainly'as a blocking layer agent on the surface of the selenium layer. After the annealing has taken place, the blocking electrode-is applied. Therefore, the creation of an artificial blocking layer on the selenium is the thing desired. However, the characteristics of the selenium rectifier in regard to the entire curcident to its operation. The rectifier can,'thererent voltage curve are changed by this process. Because of the increase of the resistance, very lowQcurreut capacity and other unfavorable characteristics result for low Voltage rectifiers having blocking layers created from oxides of foreign matter. 1 Thus such rectifiers are very unsatisfactory for the most important technical applications, such as high current rectification and the rectification of low voltages (for measurement purposes).

semi-conductor'with particularly good conductivity is created from selenium by the admixture of selenium chloride or selenium bromide (or such other material as would create selenium chloride or selenium bromide by a chemical re- In accordance with our invention, an electrical action at increased temperatures). In the latter case, a non-metallic inorganic chemical composition is preferably used or some other composition, the decomposition products of which during reaction with selenium are essentially soluble and devoid of conducting residues. Of

particular advantage are compounds of sulphur containing chlorine, preferably sulphur-chloride (SzCh). Particularly advantageous results are obtained when the total quantity of the admixtures is .01-1%, .02-3% being most advantageous. As proven by tests, through these admixtures, .the selenium already attains good conductivity after a time lapse of /4hour at particularly. low conversion temperatures. Since selenium and its admixtures form a stable compound, whose mass becomes perfectly homogeneous, the semi-cone ductor produced by this reaction has a uniform conductivity over its entire surface; Therefore the entire surface of the semi-conductor operates at a uniform current density. Therefore, changes of the semi-conductor do not occur, even after long operating periods. By employing the above-described admixtures it was found possible to convert any type of selenium into the state of the highest conductivity, even though prior to conversion it was practically non-conducting. A particular advantage was gained in that it was always possible to reproduce the desired values of conductivity,

As an example, a specimen of pure selenium was found which was absolutely unusable to admixture, and which showed a specific resistance of ohms/cm. in spite of the most careful annealing. By admixture at an annealing temperature of 170", a reduction of the specific resistance to about 100 ohms/cm. was obtained. The admixtures herein disclosed make possible the profitable production of highly efficient rectifi'ers utilizing a medium priced selenium, which would normally be wholly unsuited for this purpose, and avoid large production losses of the selenium both in its raw form and in completed rectifier discs.

However, the admixture of selenium chloride and selenium bromide produces a whole sequence of further advantages. Through the absence of foreign matters in the selenium layer, and particularly through. the absence of metallic matters, the formation of the blocking layer proceeds particularly well, easily and in a reproductible manner. During the conversion of the selenium into its crystalline form, by means of annealing, the selenium chloride or selenium bromide evaporates much faster than the selenium, and in addition is somewhat decomposed. Thus a very thin overlay of selenium is created, devoid to a great extent of the admixed material and having a much higher specific resistance than the underlying main selenium surface itself. From this thin overlay a blocking layer of pure selenium, having a very low conductivity, is created during the thermal conversion period, and thus forms the real layer from which the blocking layer is eventually formed, for example, by applying a high blocking voltage for a short time to theblocking electrode. Because of the impoverishment of the admixed substances in the high resistance surface layer, a very thin and, therefore, very high resistance blocking layer is automatically obtained during formation. Such a selenium rectifier has on the one hand a particularly great blocking action against high voltages and a very high puncture strength, and on the other hand a very good conductivity in the forward direction, even at low prior voltages. Therefore, such rectifiers have a particularly high load capacity.

For the annealing, temperatures of 150 to 170 are fully adequate. They even represent the most desirable values. It is feasible to attain the best conductivity ,5 mho per centimeter and more) with a simultaneous gradual reduction of specific resistance within the layer thickness. Above all, it is possible to control the thickness of the blocking layer, so that a wide range of variation of the blocking layer, depending upon the application of the rectifier, is possible. The above is accomplished by controlling the duration (a few minutes to one hour) of the annealing process. A particularly desirable secondary action is obtained by the above mentioned admixtures, because it is possible to apply the selenium on the underlying metal in a very thin and uniform layer. It is. therefore, possible to save on electrical resistance and to avoid breaks in the layer, which would otherwise occur because of uneven application of selenium on the base or because selenium containing foreign admixtures is used, i. e., the production losses, when compared to present known manufacturing processes, are materially lower and the quality of the rectifiers is far-superior.

In accordance with our invention, particularly great advantages are realized in the production of selenium rectifiers when utilizing a light metal base, such as aluminum. It has been shown that the adhesion of the layer and the light metal base causes difliculties when common selenium is used and that in addition high transition resistances are obtained. These have a particularly injurious influence on the efliciency of the rectifier for current-flow in the forward direction. The primary cause is the oxidation of the light metal plate. The substances added in accordance with our invention eliminate. these difiiculties and make the application of light metal as a base most favorable. The admixture of about .01% to 1% of the above-mentioned materials gives satisfactory contact surface without a substantial transition resistance, and this condition of the rectifiers does not alter. Largeramounts of admixture may cause ageing processes to appear.

For the attainment of the best possible conductivity of the semi-conducting layer, an admixture of .02% to .3% has proven particularly advantageous. In regard to the adhesion of the selenium layer to the light metal base, the best results were obtained when the entire amount of the admixtures is kept below 25%, preferably to .01% to .2%. Apparently the oxide film of the light metal base dissolves when it comes in contact with the selenium and its admixture, and an excellent adhesion of the selenium is obtained with very low transition resistances between the selenium layer and the base. .01% to .2% is proven as particularly favorable, since the reaction between the admixture and the light metal must be limited. The lowered conversion temperature (about to C.) and the shortened conversion time act in a similar manner. Also, in this manner, the reaction between the admixtures and the light, metal is limited to the most desirable degree, i. e., to such an extent that the injurious oxide film is just removed. In this manner it is not only possible to attain an extraordinary high conductivity in the selenium layer, but its full utilization for the increase of the rectifier efficiency is also possible because the creation of transition resistances at the contact face of the light metal can be avoided.

In regard to the adhesion of the selenium layer An admixture of on the light metal, and the avoidance of transition resistances between it and the deoxidized light metal base, selenium chloride and selenium bromide have proven to be the most favorable of admixtures of all the selenium halogen compounds. Aside from the fact that it is extremely diflicult or even entirely impossible for example, to obtain compounds of selenium and iodine which will be fully soluble, particularconsideration must be given to the fact that during annealing the reaction compounds must not leave a solid residue. In case of the invention, chlorine and bromine are used which are gaseous and leave no solid residue, as would be the case if iodine were used. Besides, chlorides and bromides are much cheaper to manufacture than iodine compounds. The difiiculties surrounding the production of iodine compounds of the selenium and also their preservation in the selenium, is the primary reason that only comparatively poor conductivity can be obtained with iodine admixtures, while with the aid of selenium chloride and selenium bromide it is possible to increase the conductivity to at least mho per centimeter. Iodine, which does not enter into a compound with selenium, acts only as an imbedded material, whereby dangerous shunting paths are created. In any case, only a seemingly increased conductivity of the selenium becomes apparent.

As compared to the use of selenium, which has been treated with chemically free chlorine or bromine, the admixture of selenium chloride or selenium bromide has, according to the invention,

the decisive advantage of being a practically usable production method, in that the raw selenium for application on a base is always ob- .tained in a uniform and homogeneous composition. The selenium chloride or selenium bromide nium can be produced by easy simple chemical processes, for example a precipitation process, and has a well defined chemical composition.

In accordance with the invention, the use of lo'admixtures do not only have a desirable effect upon the light-metal-selenium-rectifier, but also for selenium rectifiers utilizing other bases such as iron or carbon.

What is claimed is:

1. An electrical semi conductor of selenium, in particular for use in dry rectifiers in which the selenium contains a substance from the group which consists of selenium chloride and selenium before admixture with normal commercial sele- 

